Short Shot During 1st Stage Fill

I recently joined in on a discussion in a Linked-In forum. I addressed one of the concerns, which I will add here…

Is the better have a 98% filled only parts with lower speed (not very close to knee of rheology curve) or 80% with higher speed or use multiple speeds fast to slow to get 98%?

My Comments
The theoretical point with respect to processing is that you have a short shot during 1st stage fill, assuming it is a velocity controlled machine with adequate injection pressure. The purpose of this practice is to use second stage packing to complete filling and compensate for normal variation in material viscosity. A faster injection speed tends to promote a more consistent process since you are processing after shear thinning occurs, but you should not exceed the machine’s capability to maintain a consistent process.
In many machines, typically older ones, you need to use a profile near the end of fill to counteract complications such as overshoot, undersized 2nd stage pump, avoid a pressure-limited process, or slow responding microprocessors. Such numbers are general rules of thumb which often distract the processor from the intent.

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